1. Field of the Invention
This invention relates to microwave signal couplings and in particular to beam-type microwave couplings between a stationary feed and a movable director. The invention herein applies primarily to microwave applications using dish reflectors wherein the wavelength of the microwave energy is small compared to the size of the dish reflector.
In recent years attention has been given to development of microwave reflector systems wherein the wavelength is small compared to the size of the reflector. In such instances, microwave signal propagation can be treated much like traditional optical signal propagation with mirrored reflectors, lenses and the like in so-called beam waveguide systems.
Microwave antennas requiring only a limited range of motion, for example less than a few tens of degrees, are frequently employed in satellite communications for earth stations or mounted on spacecraft. Signal losses are particularly critical in spacecraft applications, since communications must normally take place with a limited amount of power over great distances.
It is generally preferably to locate microwave electronic equipment on the stationary side of the motional interface of a steerable microwave antenna and to transmit the microwave signals across the motional interface. For example, the feed may be mounted on the base and the signals may be transmitted to a focal point through an open or beam-type waveguide. The rotary interface is accomplished by beaming or propagating the signals between two reflectors which are each mounted on opposing sides of the motional interface.
2. Description of the Prior Art
In prior art beam waveguide feed systems, it has generally been taught that the axis of rotation be parallel and generally along the guide axis or axis of signal propagation. Therefore, to obtain rotation having a maximal degree of freedom, a pair of reflectors is required for each rotary axis.
A representative example of a prior art beam feed in U.S. Pat. No. 4,186,402 to Mizusawa et al. The Mizusawa et al. patent discloses a steerable microwave antenna in which microwave energy is conveyed between a movable aerial portion and a fixed portion containing the primary feed. The beam waveguide consists of four reflectors which together with the moveable aerial portion are rotatable relative to the fixed portion about one axis along the guide axis and rotatable about a second orthogonal axis along the guide axis.
U.S. Pat. No. 4,044,361 to Yokoi et al. discloses a satellite tracking antenna including a main reflector, a sub-reflector and beam waveguide reflectors wherein one beam waveguide reflector is adapted to be shifted transversely to move the feeding point of the sub-reflector. In addition, the tracking antenna is rotatable about an axis along the guide axis.
U.S. Pat. No. 4,062,018 to Yokoi et al. discloses a scanning antenna with a moveable beam waveguide feed similar in structure and operation to the above-described Yokoi et al. patent. Various beam waveguide structures are disclosed. Beam waveguide reflectors are translated in axes along the guide axis.
U.S. Pat. No. 3,845,483 to Soma et al. discloses a beam waveguide feed wherein a rotatable microwave feed portion is interposed between the microwave source and the antenna. The reflector is independently rotatable in two transverse directions along the guide axis.
Two other patents were uncovered as a consequence of a search of the U.S. Patent and Trademark Office records. In the first, U.S. Pat. No. 3,680,141 to Karikomi, there is shown an antenna system with a movable plane reflector and a movable sub-reflector which are used to deflect radiated waves without moving a main reflector. This system is distinguishable in that the main reflector does not move and the sub-reflector moves with respect to the main reflector. U.S. Pat. No. 3,795,003 to Meeke et al. discloses another example of an antenna system with a rotatable feed for use in scanning in a turnstyle scanner. A reflector is used for scanning and switching among feeds. The main reflector in such an antenna system is not intended to be rotated.
General background material in waveguide rotary and swivel joints is found in Sommers et al. "Beam-Waveguide Feed", Microwave Journal, November 1975, page 51.